Zoom lens and image projection apparatus having the same

ABSTRACT

A zoom lens of the invention has three lens groups. In the zoom lens, magnification change is performed by moving only one lens group among the three lens groups, and focus adjustments is performed by moving one of a first lens group and a second lens group counted from an enlargement conjugate side among the three lens groups. In addition, the zoom lens is approximately telecentric on reduction conjugate side thereof.

This application is a divisional of prior application Ser. No.11/099,905, filed Apr. 5, 2007, which is incorporated by referenceherein in its entirety as if fully set forth.

This application claims the right of priority under 35 U.S.C. § 119based on Japanese Patent Application No. 2004-112550, filed on Apr. 6,2004, which is also incorporated by reference herein in its entirety asif fully set forth.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens and an image projectionapparatus having the zoom lens which are, for instance, suitable for aliquid crystal projector apparatus that magnifies and throws an imagedisplayed by an image display element, such as a light valve, onto ascreen.

2. Related Background Art

Conventionally, a liquid crystal projector has been widely used in aconference, presentation, or the like as an image projection apparatusthat throws an image of a personal computer or the like onto a largescreen, thereby allowing people to view the image.

There is a demand that an optical system applied to the liquid crystalprojector is capable of projecting an image whose brightness is uniformand luminance is high, so it is desired that a pupil on a liquid crystaldisplay element (reduction conjugate) side is located at infinity, thatis, the pupil is so-called telecentric.

Conventionally, as a large-aperture and high-resolution projection lensfor the liquid crystal projector, various zoom lenses have been proposedwhich are capable of compensating for various aberrations by arrangingfive or six lens groups under an appropriate refractive power condition(Japanese Patent Application Laid-Open No. 2004-70306).

Among the zoom lenses, a projection zoom lens for a liquid crystalprojector is known which is composed of six lens groups that are a firstlens group having negative refractive power, a second lens group havingpositive refractive power, a third lens group having positive refractivepower, fourth lens group having negative refractive power, a fifth lensgroup having positive refractive power, and a sixth lens group havingpositive refractive power that are arranged in this order from anenlargement conjugate side that is a screen side to a reductionconjugate side that is a display image side, with the second lens group,the third lens group, and the fifth lens group being moved at the timeof magnification changing (Japanese Patent Application Laid-Open No.2001-108900).

Also, an optical-correction-type zoom lens is known which is composed oftwo groups that are a first lens group having negative refractive powerand a second lens group having positive refractive power that arearranged in this order from an object side (enlargement conjugate side)to an image side (reduction conjugate side), with magnification changingbeing performed by moving only the second lens group (U.S. Pat. No.5,745,303).

Generally, in a liquid crystal projector that uses a liquid crystalpanel as an image display element, it is important that a brightprojection optical system is used and no shading exists. For instance,when the projection optical system is not image-side telecentric,brightness unevenness occurs and image quality is adversely affected. Inaddition, in recent years, there has been a demand for miniaturizationof a projection lens resulting from miniaturization of the whole of aprojector apparatus.

A multi-group optical system where a projection optical system iscomposed of multiple lens groups and the multiple lens groups are movedat the time of magnification changing is advantageous in terms ofaberration compensation, but there is generally a problem in that theconstruction of the projection optical system becomes complicated and anincrease in size is inevitable.

Also, the zoom lens proposed in U.S. Pat. No. 5,745,303 B as a simplelens construction is a zoom lens for a compact camera. Therefore, thereduction-side telecentricity of the zoom lens is not sufficient for aliquid crystal projector. In addition, the F-value of the zoom lens isaround 10, so it is not necessarily possible to provide brightnesssufficient for the projector.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentionedcircumstances and an aspect of the invention is a zoom lens includingthree lens groups in which magnification change is performed by movingonly one lens group among the three lens groups, focus adjustments ofthe zoom lens is performed by moving one of a first lens group and asecond lens group counted from an enlargement conjugate side among thethree lens groups, and the zoom lens is approximately telecentric onreduction conjugate side of the zoom lens.

Further, another aspect of the invention is an image projectionapparatus including an image display element and a zoom lens forprojecting light from the image display element, in which the zoom lenscomprises three lens groups, magnification change is performed by movingonly one lens group among the three lens groups, focus adjustments ofthe zoom lens is performed by moving one of a first lens group and asecond lens group counted from an enlargement conjugate side among thethree lens groups, and the zoom lens is approximately telecentric onreduction conjugate side of the zoom lens.

Further, another aspect of the invention is a zoom lens including: afirst lens group having negative refractive power; a second lens grouphaving positive refractive power; and a third lens group having positiverefractive power, in which the first lens group, the second lens group,and the third lens group are located in this order from an enlargementconjugate side, and in which magnification changing is performed bymoving only the second lens group and focus adjustment is performed bymoving only the first lens group.

Further, another aspect of the invention is an image projectionapparatus including: an image display element; and a zoom lens forprojecting light from the image display element, in which zoom lenscomprises a first lens group having negative refractive power, a secondlens group having positive refractive power, and a third lens grouphaving positive refractive power, which are located in this order froman enlargement conjugate side, and in which magnification changing isperformed by moving only the second lens group and focus adjustment isperformed by moving only the first lens group.

Further, another aspect of the invention is a zoom lens including: afirst lens group having negative refractive power; a second lens grouphaving positive refractive power; a third lens group having positiverefractive power; and a fourth lens group having positive refractivepower, in which the first lens group, the second lens group, the thirdlens group, and the fourth lens group are located in this order from anenlargement conjugate side, and in which magnification changing isperformed by moving only the third lens group and focus adjustment isperformed by moving only the second lens group.

Further, another aspect of the present invention is an image projectionapparatus including: an image display element; and a zoom lens forprojecting light from the image display element, in which zoom lenscomprises a first lens group having negative refractive power, a secondlens group having positive refractive power, a third lens group havingpositive refractive power, and a fourth lens group having positiverefractive power, which are located in this order from an enlargementconjugate side, and in which magnification changing is performed bymoving only the third lens group and focus adjustment is performed bymoving only the second lens group.

Further, another aspect of the invention is an image projectionapparatus including: an image display element; and a zoom lens thatprojects light from the image display element, in which: the imagedisplay element is disposed on a reduction conjugate side of the zoomlens; the zoom lens includes a plurality of lens groups; a pupil on thereduction conjugate side of the zoom lens is located at approximatelyinfinity; magnification changing is performed by moving only one lensgroup for magnification changing among the plurality of lens groups onan optical axis; and when an imaging magnification at a magnificationchanging position at a wide angle end of the lens group formagnification changing is “•_(VW)” a focal distance of the lens groupfor magnification changing is “f_(V)” and a lateral magnification, amagnification changing ratio, an F-value in an intermediate region at atime of magnification changing, and a minimum diameter of a circle ofconfusion of a last lens group are respectively “•_(r)”, “Z”, “F_(m)”,and “•”, following expressions are satisfied:−1.2<β_(VW)<−0.82F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(T) ² /√{square rootover (Z)}>0

Further, another aspect of the invention is a zoom lens including: threeor more lens groups, in which: magnification changing is performed bymoving only one lens group for magnification changing among the three ormore lens groups in an optical axis direction of the zoom lens; and whenan imaging magnification at a magnification changing position at a wideangle end of the lens group for magnification changing is “•_(VW)”, afocal distance of the lens group for magnification changing is “f_(V)”,and a lateral magnification, a magnification changing ratio, an F-valuein an intermediate region at a time of the magnification changing, and aminimum diameter of a circle of confusion of a last lens group arerespectively “•_(r)”, “Z”, “F_(m)”, and “•”, following expressions aresatisfied:−1.2<β_(VW)<−0.82F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(T) ² /√{square rootover (Z)}>0

Further, another aspect of the invention is an image projectionapparatus including: an image display element; and a zoom lens thatprojects light from the image display element, in which: the imagedisplay element is disposed on a reduction conjugate side of the zoomlens; the zoom lens includes three or more lens groups; magnificationchanging is performed by moving only one lens group for magnificationchanging among the three or more lens groups; and when an imagingmagnification at a magnification changing position at a wide angle endof the lens group for magnification changing is “•_(VW)”, a focaldistance of the lens group for magnification changing is “f_(V)”, and alateral magnification, a magnification changing ratio, an F-value in anintermediate region at a time of magnification changing, and a minimumdiameter of a circle of confusion of a last lens group are respectively“•_(r)”, “Z”, “F_(m)”, and “•”, following expressions are satisfied:−1.2<β_(VW)<−0.82F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(T) ² /√{square rootover (Z)}>0

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lens cross-sectional view showing both cases of a wide angleend and a tele photo end according to a first embodiment of the presentinvention;

FIGS. 2A and 2B are respectively aberration diagrams at the wide angleend and the tele photo end where an object distance is set at 2.35 m anda zoom lens according to a first numerical embodiment is expressed inunits of mm;

FIG. 3 is a lens cross-sectional view showing both cases of a wide angleend and a tele photo end according to a second embodiment of the presentinvention;

FIGS. 4A and 4B are respectively aberration diagrams at the wide angleend and the tele photo end where an object distance is set at 2.35 m anda zoom lens according to a second numerical embodiment is expressed inunits of mm;

FIG. 5 is a lens cross-sectional view showing both cases of a wide angleend and a tele photo end according to a third embodiment of the presentinvention;

FIGS. 6A and 6B are respectively aberration diagrams at the wide angleend and the tele photo end where an object distance is set at 2.35 m anda zoom lens according to a third numerical embodiment is expressed inunits of mm; and

FIG. 7 is a schematic view of the main portion of an image projectionapparatus according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The zoom lens and the image projection apparatus according to thepresent invention will now be described below.

The zoom lens according to the present invention is a zoom lens havingmultiple lens groups where a pupil position on a reduction conjugateside is located at approximately infinity and magnification changing isperformed by moving only one lens group for magnification changing, outof the multiple lens groups, on an optical axis. Note that thedescription in which a pupil position on a reduction conjugate side islocated an approximately infinity, means that the zoom lens isapproximately telecentric on reduction conjugate side thereof.

Also, the zoom lens according to the present invention is composed ofthree or more lens groups and magnification changing is performed bymoving only one lens group for magnification changing, out of the threeor more lens groups, on the optical axis.

Here, the pupil position on the reduction conjugate side is located atapproximately infinity. That is, the pupil position is substantiallytelecentric on a reduction side.

Also, focus adjustment is performed by moving a lens group, which is thefirst lens group or the second lens group when counted in a directionfrom the enlargement conjugate side to the reduction conjugate side, onthe optical axis. In particular, the first lens group has negativerefractive power.

As to magnification changing, the first lens group (lens group on themost enlargement conjugate side) and the last lens group (lens group onthe most reduction conjugate side) of the zoom lens according to theembodiment are fixed with respect to a conjugate surface on thereduction side at the time of the magnification changing. The last lensgroup which is on the most reduction conjugate side, is composed of onepositive lens (as a matter of course, when positive refractive power ispossessed, the last lens group may be composed of two or more lenses).Also, the lens group for magnification changing is composed of two ormore positive lenses and one or more negative lenses.

In the zoom lens according to the present invention, one or moresurfaces that each have an aspherical surface shape are disposed andeach lens having such a surface in the aspherical surface shape is aplastic lens. Also, the zoom lens according to the present invention hasone or more lenses that are each made of glass whose Abbe number is 80or more.

Also, the zoom lens according to the present invention has a first lensgroup having negative refractive power, a second lens group havingpositive refractive power, and a third lens group having positiverefractive power in this order from the enlargement conjugate side tothe reduction conjugate side and performs magnification changing bymoving only the second lens group on the optical axis. Here, the secondlens group is positioned on an enlargement conjugate side at amagnification changing position at a tele photo end with respect to amagnification changing position at a wide angle end.

Here, when the imaging magnification at the magnification changingposition at the wide angle end of the second lens group is referred toas “•_(VW)”, the following expression is satisfied:−1.2<β_(VW)<−0.8

In addition, when the focal distance of the second lens group isreferred to as “f_(V)” and the lateral magnification, the magnificationchanging ratio, the F-value in an intermediate region at the time ofmagnification changing, and the minimum diameter of a circle ofconfusion of the third lens group are respectively referred to as“•_(r)>”, “Z”, “F_(m)”, and “•”, the following expression is satisfied:2F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(T) ² /√{square rootover (Z)}>0

In the zoom lens having the three-group construction described above,focus adjustment is performed by moving the first lens group in anoptical axis direction.

In addition, the zoom lens according to the present invention has afirst lens group having negative refractive power, a second lens grouphaving positive refractive power, a third lens group having positiverefractive power, and a fourth lens group having positive refractivepower in this order from the enlargement conjugate side to the reductionconjugate side and performs magnification changing by moving only thethird lens group in the optical axis direction. Here, the third lensgroup is positioned on the enlargement conjugate side at themagnification changing position at the tele photo end with respect tothe magnification changing position at the wide angle end.

Here, when the imaging magnification at the magnification changingposition at the wide angle end of the third lens group is referred to as“•_(VW)”, the following expression is satisfied:−1.2<β_(VW)<−0.8

In addition, when the focal distance of the third lens group is referredto as “f_(V)” and the lateral magnification, the magnification changingratio, the F-value in an intermediate region at the time ofmagnification changing, and the minimum diameter of a circle ofconfusion of the fourth lens group are respectively referred to as“•_(r)”, “Z”, “F_(m)”, and “•”, the following expression is satisfied:2F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(T) ² /√{square rootover (Z)}>0

In the zoom lens having the four-group construction described above,focus adjustment is performed by moving the second lens group on theoptical axis.

The zoom lens according to the present invention has an aperturediaphragm and when the synthesized focal distance of the lens systemfrom the aperture diaphragm to the reduction conjugate side is referredto as “f_(r)” and the distance from the diaphragm to the principal pointposition on the enlargement conjugate side of the lens system on thereduction conjugate side is referred to as “L”, the following expressionis satisfied:0.45<L/f _(r)<1.1

In addition, when the paraxial back focuses at the magnificationchanging positions at the wide angle end and the tele photo end arerespectively referred to as “BF_(W)” and “BF_(T)”, the F-value at thetele photo end is referred to as “F_(T)”, and the minimum diameter of acircle of confusion is referred to as “•”, the following expression issatisfied:2•F _(T) ••−|BF _(W) −BF _(T)|>0

Also, on the enlargement conjugate side and the reduction conjugate sidewith the aperture diaphragm described above in-between, one or moresurfaces that each have an aspherical surface shape is possessed.

Also, a negative lens LN including a surface having an asphericalsurface shape and a positive lens LP including a surface having anaspherical surface shape are respectively provided on the enlargementconjugate side and the reduction conjugate side with the aperturediaphragm in-between. Further, the surface in the aspherical surfaceshape of the negative lens LN has a concave shape whose concave degreeis reduced in a direction from a lens center to a lens peripheralportion and the surface in the aspherical surface shape of the positivelens LP has a convex shape whose convex degree is reduced in a directionfrom a lens center to a lens peripheral portion. Still further, thenegative lens LN and the positive lens LP are each made of plastic.

Also, the zoom lens according to the present invention is a zoom lenshaving multiple lens groups where a pupil position on the reductionconjugate side is located at approximately infinity, magnificationchanging is performed by moving only one lens group for magnificationchanging, out of the multiple lens groups, on the optical axis, and whenthe imaging magnification at the magnification changing position at thewide angle end of the lens group for magnification changing is referredto as “•_(VW)”, the focal distance of the lens group for magnificationchanging is referred to as “f_(V)”, and the lateral magnification, themagnification changing ratio, the F-value in an intermediate region atthe time of magnification changing, and the minimum diameter of a circleof confusion of the last lens group are respectively referred to as“•_(r)”, “Z”, “F_(m)”, and “•”, the following expressions are satisfied:−1.2<β_(VW)<−0.82F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(T) ² /√{square rootover (Z)}>0

Also, the zoom lens according to the present invention is composed ofthree or more lens groups, where magnification changing is performed bymoving only one lens group for magnification changing, out of themultiple lens groups, on the optical axis and when the imagingmagnification at the magnification changing position at the wide angleend of the lens group for magnification changing is referred to as“•_(VW)”, the focal distance of the lens group for magnificationchanging is referred to as “f_(V)”, and the lateral magnification, themagnification changing ratio, the F-value in an intermediate region atthe time of magnification changing, and the minimum diameter of a circleof confusion of the last lens group are respectively referred to as“•_(r)”, “Z”, “F_(m)”, and “•”, the following expressions are satisfied:−1.2<β_(VW)<−0.82F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(T) ² /√{square rootover (Z)}>0

Also, the image projection apparatus according to the present inventionincludes an image display element (light modulation element such as aliquid crystal panel) and any one of the zoom lenses described above andthrows an original image displayed by the image display element onto aprojection target screen using the zoom lens. Here, the minimum diameterof a circle of confusion is set twice as large as the pixel pitch of theimage display element.

Also, when the imaging magnification at the magnification changingposition at the wide angle end of the lens group for magnificationchanging is referred to as “•_(VW)”, the focal distance of the lensgroup for magnification changing is referred to as “f_(V)”, the lateralmagnification, the magnification changing ratio, and the F-value in anintermediate region at the time of magnification changing of the lastlens group are respectively referred to as “•_(r)”, “Z”, “F_(m)”, andthe minimum diameter of a circle of confusion “•” of the zoom lens isset twice as large as the pixel pitch of the image display element, thefollowing expressions are satisfied:−1.2<β_(VW)<−0.82F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(T) ² /√{square rootover (Z)}>0

Hereinafter, embodiments of the zoom lens and the image projectionapparatus having the same according to the present invention will bedescribed in a concrete manner with reference to the drawings.

FIG. 1 is a schematic diagram of a main portion showing both cases of awide angle end (short focal distance) and a tele photo end (long focaldistance) of an image projection apparatus (liquid crystal videoprojector) that uses a zoom lens according to a first embodiment of thepresent invention.

FIGS. 2A and 2B are respectively aberration diagrams at the wide angleend (short focal distance) and the tele photo end (long focal distance)where an object distance (distance from the first lens group) is set at2.35 m and numerical values in a first numerical embodiment to bedescribed later corresponding to the first embodiment of the presentinvention are expressed in units of mm.

FIG. 3 is a schematic diagram of a main portion showing both cases of awide angle end (short focal distance) and a tele photo end (long focaldistance) of an image projection apparatus (liquid crystal videoprojector) that uses a zoom lens according to a second embodiment of thepresent invention.

FIGS. 4A and 4B are respectively aberration diagrams at the wide angleend (short focal distance) and the tele photo end (long focal distance)where an object distance (distance from the first lens group) is set at2.35 m and numerical values in a second numerical embodiment to bedescribed later corresponding to the second embodiment of the presentinvention are expressed in units of mm.

FIG. 5 is a schematic diagram of a main portion showing both cases of awide angle end (short focal distance) and a tele photo end (long focaldistance) of an image projection apparatus (liquid crystal videoprojector) that uses a zoom lens according to a third embodiment of thepresent invention.

FIGS. 6A and 6B are respectively aberration diagrams at the wide angleend (short focal distance) and the tele photo end (long focal distance)where an object distance (distance from the first lens group) is set at2.35 m and numerical values in a third numerical embodiment to bedescribed later corresponding to the third embodiment of the presentinvention are expressed in units of mm.

FIG. 7 is a schematic diagram of a main portion of a color liquidcrystal projector to which the zoom lens according to the presentinvention is applied.

In each of the image projection apparatuses according to the first tothird embodiments shown in FIGS. 1, 3, and 5, an original image(throwing target image) displayed on a liquid crystal panel LCD or thelike is magnified and thrown onto a screen surface S using the zoom lens(throwing lens, projection lens) PL.

In the drawings, reference symbol S denotes a screen surface (throwingsurface) and reference symbol LCD denotes an original image (throwingtarget image) on a liquid crystal panel (liquid crystal display element)or the like. The screen surface S and the original image LCD are in aconjugate relationship and, generally, the screen surface S correspondsto a conjugate point having a long distance (enlargement conjugate side)and the original image LCD corresponds to a conjugate point having ashort distance (reduction conjugate side).

Reference symbol GB denotes a glass block composed of, for instance, acolor synthesis prism, a polarizing filter, and a color filter.

The zoom lens PL is fitted to the main body (not shown) of the liquidcrystal video projector through a connection member (not shown). Aliquid crystal display element LCD side following the glass block GB iscontained in the projector main body.

Reference symbol Li denotes the ith lens group when counted in adirection from the enlargement conjugate side to the reduction conjugateside.

In the first and second embodiments respectively shown in FIGS. 1 and 3,reference symbol L1 denotes the first lens group having negativerefractive power, reference symbol L2 denotes the second lens grouphaving positive refractive power, and reference symbol L3 denotes thethird lens group having positive refractive power.

In the third embodiment shown in FIG. 5, reference symbol L1 denotes thefirst lens group having negative refractive power, reference symbol L2denotes the second lens group having positive refractive power,reference symbol L3 denotes the third lens group having positiverefractive power, and reference symbol L4 denotes the fourth lens grouphaving positive refractive power. Reference symbol SP denotes adiaphragm.

The zoom lens in each embodiment is a so-called optical-correction-typezoom lens where one lens group Va is moved on the optical axis at thetime of magnification changing from the wide angle end to a zoomposition at the tele photo end.

The lens group Va is the second lens group L2 in the first and secondembodiments shown in FIGS. 1 and 3 and is the third lens group L3 in thethird embodiment shown in FIG. 5 and the magnification changing from thewide angle end to the tele photo end is performed by moving the lensgroup Va from the reduction conjugate side to the enlargement conjugateside as indicated by arrows.

With respect to the magnification changing position at the wide angleend, at the magnification changing position at the tele photo end, thelens group Va is positioned on an enlargement conjugate side.

The last lens group optically acts as a field lens, by which the pupilposition on the reduction conjugate side is located at approximatelyinfinity (when the pupil position is located at approximately infinity,it is not necessarily required that the pupil position is located atinfinity and it is sufficient that a range expressed by ConditionalExpression (2) given below is satisfied). Lens surfaces in the entiresystem include one or more surfaces that each have an aspherical surfaceshape. Lenses that each have such a surface in the aspherical surfaceshape include lenses produced through molding of plastic.

In each embodiment, a retrofocus-type construction is used in which thelens group on the most enlargement conjugate side has negativerefractive power and the lens group on the most reduction conjugate sidehas positive refractive power, thereby securing desired back focus andmakes it easy to realize a wide angle. It is preferable that a three- ormore-group construction be used and it is more preferable that aconstruction having a lens group arrangement of negative, positive, andpositive refractive power be used because it becomes possible to securetelecentric performance and to favorably compensate for variousaberrations in the simplest manner.

At the time of magnification changing, the lens group on the mostenlargement conjugate side and the lens group on the most reductionconjugate side are fixed with respect to the conjugate surface on thereduction side, by which a zoom lens, whose lens overall length at thetime of magnification changing is constant, is obtained and centroidmovement of lens groups, whose diameters are large, is eliminated, so itbecomes possible to realize stabilized lens holding at the time ofmounting to a liquid crystal throwing apparatus.

It should be noted here that the lens group on the most reductionconjugate side is required to have positive refractive power in order torealize telecentric performance, however the lens group is composed ofonly one positive lens, thereby achieving further weight reduction.

In addition, in some cases, it is possible to use the lens group on themost reduction conjugate side as a back focus adjustment group, so thelens group uses a simple construction where only one positive lens isused.

The lens group Va that moves at the time of magnification changing iscomposed of at least two positive lenses and at least one negative lens.The lens group Va is a lens group that is solely moved for magnificationchanging and a construction is selected in which even when magnificationis changed in the vicinity of equal-magnification as a result ofmovement of the lens group Va, there will not occur any large aberrationvariations. Therefore, the lens group Va is constructed using at leasttwo positive lenses for sharing of positive refractive power andreduction of aberration occurrence and using one negative lens forsufficient compensation for aberrations that occurred at the positivelenses.

Also, in order to favorably compensate for distortion, coma aberrations,and the like that are generally ascribable to an asymmetric powerarrangement and are unique to a retrofocus-type lens, one or more lenseseach having an aspherical surface shape are disposed on each of theenlargement conjugate side and the reduction conjugate side with theaperture diaphragm SP in-between. Preferably, as to the lenses eachhaving the aspherical surface shape, the lens in the aspherical surfaceshape on the enlargement conjugate side is a negative lens and the lensin the aspherical surface shape on the reduction conjugate side is apositive lens. In addition, as to the surface shape, the negative lensin the aspherical surface shape on the enlargement conjugate side withthe aperture diaphragm SP in-between has the aspherical surface shape onits concave surface, with a concave degree of the aspherical surfaceshape being reduced in a direction from the lens center to theperipheral portion.

When the positive lens in the aspherical surface shape on the reductionconjugate side has the aspherical surface shape on its convex surfaceand a convex degree of the aspherical surface shape is reduced in adirection from the lens center to the peripheral portion, it becomespossible to further reduce various aberrations that occur at eachrefractive surface. As to the arrangement of the aspherical surfaces, interms of aberration compensation, it is effective that at least oneaspherical surface is provided at a position, such as in the first lensgroup, that is as far as possible from the diaphragm SP.

In addition, the lens in the aspherical surface shape is producedthrough molding of plastic, thereby realizing further cost reduction andweight reduction of the optical system in addition to the weightreduction through the reduction of the number of construction elements.

Also, from the viewpoint of chromatic aberration compensation, anoptical system with less color drift in a visible region is realizedusing at least one lens made of glass whose Abbe number is 80 or more.

Focusing is performed by moving the first lens group L1 or the secondlens group counted from the enlargement conjugate side. When the imagingmagnification at the magnification changing position at the wide angleend of the magnification changing lens group (the second lens group L2in the first and second embodiments and the third lens group L3 in thethird embodiment) is referred to as “•_(VW)”, the following expressionis satisfied:−1.2<β_(VW)<−0.8  (1)

When a synthesized focal distance of the lens system from the aperturediaphragm SP to the reduction conjugate side is referred to as “f_(r)”and a distance from the diaphragm SP to the principal point position onthe enlargement conjugate side of the lens system on the reductionconjugate side is referred to as “L”, the following expression issatisfied:0.45<L/f _(r)<1.1  (2)

When the focal distance of the lens group Va is referred to as “f_(V)”and the lateral magnification, the magnification changing ratio, theF-value in an intermediate region at the time of magnification changing,and the minimum diameter of a circle of confusion of the third lensgroup in the first and second embodiments and the fourth lens group L4in the third embodiment are respectively referred to as “•_(r)”, “Z”,“F_(m)”, and “•”, the following expression is satisfied:2F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(T) ² /√{square rootover (Z)}>0  (3)

When the paraxial back focuses at the magnification changing positionsat the wide angle end and the tele photo end are respectively referredto as “BF_(W)” and “BF_(T)”, the F-value at the tele photo end isreferred to as “F_(T)”, and the minimum diameter of a circle ofconfusion is referred to as “•”, the following expression is satisfied:2•F _(T) ••−|BF _(W) −BF _(T)|>0  (4)

Also, the following expression is satisfied:1.5<F_(m)<2.2  (5)

Next, the technical meaning of each conditional expression describedabove will be described.

When the lower limit of Conditional Expression (1) is exceeded, at thetime of magnification changing from the wide angle end to the tele photoend, an unfavorable situation occurs in which as to the focal surface,movement in the backward direction with respect to the lens groups istoo large. Also, when the upper limit is conversely exceeded, anunfavorable situation occurs in which movement in the frontwarddirection with respect to the lens groups is too large.

When the zoom lens according to the present invention is applied to athrowing system for a liquid crystal projector, it is preferable thatthe pupil position be arranged at a position that is as far as possiblefrom the reduction conjugate side. The same applies to an optical systemthat uses an image pickup element (CCD) or the like. More specifically,it is preferable that Conditional Expression (2) be satisfied.

With a construction outside the range expressed by ConditionalExpression (2), it is impossible to obtain favorable telecentricperformance. When a zoom lens that does not have favorable telecentricperformance is applied to a throwing system for a liquid crystalprojector or the like, an unfavorable situation occurs in whichilluminance lowering, an in-screen color unevenness, or the like occurson the periphery of the screen. Also, when focus adjustment is performedby moving a lens group on the enlargement conjugate side with respect toa magnification changing portion, it becomes possible to ensure aconstant focal position for the entire magnification changing regioneven when an object-image distance is changed.

As to the magnification changing lens group, when it is possible toconstruct the lens system with one component, it becomes possible tosimplify a lens-barrel structure and the like, which enables weightreduction through a reduction of the number of construction elements orthe like.

The zoom lens in each embodiment is an optical-correction-type zoomlens, so from the viewpoint of a focus movement amount at the time ofmagnification changing, when the focal distance of the magnificationchanging lens group is referred to as “f_(V)” and the lateralmagnification, the magnification changing ratio, the F-value in anintermediate region, and the minimum diameter of a circle of confusionof the last (third) lens group are respectively referred to as “•_(r)”,“Z”, “F_(m)”, and “•” and when consideration is given to a conditionwhere the focal position coincides at the wide angle end and the telephoto end, it is possible to obtain the maximum focus displacementamount •_(m) of displacements in the intermediate region at the time ofmagnification changing by newly multiplying the expression in thetwo-group zoom lens described in Japanese Patent Application Laid-OpenNo. H07-333500 by the squares of the lateral magnifications of the threegroups in the case of this embodiment, so it is possible to paraxiallycalculate the maximum focus displacement amount •_(m) in the mannergiven below.Δ_(n) =|−f _(V)·(1−√{square root over (Z)})²|·β_(r) ² /√{square rootover (Z)}

Also, the depth of focus • is expressed by the product of the F-value ofthe lens and the permissible diameter of a circle of confusion • andtherefore is defined as follows.δ=F _(m)·εWhen •_(m) is allowed fully as to the depth on both sides of themagnification changing range and “2•>•_(m)”, even when a focusdisplacement occurs, it is contained within the depth of focus, so itcan be seen that it is sufficient that Conditional Expression (3) besatisfied.

A situation where Conditional Expression (3) takes a negative value isnot preferable because, in this case, the focus displacement amount atthe time of magnification changing becomes greater than the depth offocus of the optical system.

It should be noted here that in each embodiment, it is preferable thatthe minimum diameter of a circle of confusion • be around twice as largeas the pixel pitch in an image pickup device using a CCD, a projectorusing a liquid crystal panel, or the like. A situation where the minimumdiameter of a circle of confusion • is increased from that level is notpreferable because, in this case, a blurring diameter is increased.

Also, when the paraxial back focus is displaced at the zoom positions atthe wide angle end and the tele photo end, it is preferable thatConditional Expression (4) be satisfied because in this case, it alsobecomes possible to suppress a defocusing amount so as to fall withinthe depth.

Conditional Expression (5) is an expression for achieving an increase inluminance. Also, Conditional Expression (5) is an expression forsuppression within the depth of focus during entire magnificationchanging as the optical-correction-type zoom lens.

A situation where the upper limit value or the lower limit value ofConditional Expression (5) is exceeded is not preferable becausebrightness becomes insufficient or the depth of focus is exceeded, forinstance, and therefore the quality of a photographed image or picked-upimage is lowered.

It should be noted here that in each embodiment, it is more preferablethat numerical value ranges of Conditional Expressions (1), (2), and (5)be set as follows.−1.0<β_(VW)<−0.8  (1a)0.5<L/f _(r)<1.0  (2a)1.6<f_(m)<2.1  (5a)

Also, it is still more preferable that the lower limit value ofConditional Expression (3) be set at 0.04. In addition, the lower limitvalue of Conditional Expression (4) may be set at 0.03.

Further, the numerical value range of Conditional Expression (2) may benarrowed as follows.0.5<L/f _(r)<0.65  (2a)

Next, features of the lens construction in each embodiment will bedescribed. As is apparent from the following description of theembodiment, the lens group in the embodiment is not required to havemultiple lenses and it is sufficient that the lens group have at leastone lens.

FIRST EMBODIMENT

The first embodiment shown in FIG. 1 has a three-group constructioncomposed of the lens group L1 having negative refractive power, the lensgroup L2 having positive refractive power, and the lens group L3 havingpositive refractive power that are arranged in this order from theenlargement conjugate side to the reduction conjugate side. When anobject-image distance is changed, focus adjustment is performed bymoving the first lens group L1.

At the time of magnification changing from the wide angle end to thetele photo end, the first lens group L1 and the third lens group L3 arefixed and have constant total lengths and only the second lens group L2is independently moved toward the enlargement conjugate side. Also, inorder to ensure illuminance on the screen S, a multilayer coat isapplied to each lens surface.

The first lens group L1 has a three-lens construction including anegative lens G11, a negative lens G12, and a positive lens G13 from theenlargement conjugate side. By designing the first lens G11 disposed onthe most enlargement conjugate side using a negative lens, an apparentpupil position is set on the enlargement conjugate side and the diameterof a front lens is reduced. Also, the first lens G11 is a lens which ismade of a plastic material and whose both surfaces each have anaspherical surface shape, thereby making it possible to compensate fordistortion and the like with efficiency.

Also, the second lens group L2 functions as a magnification changinglens group. In this embodiment, the second lens group L2 includes apositive lens G21, a diaphragm SP, a negative lens G22, a positive lensG23, and a positive lens G24 in this order from the enlargementconjugate side, and is a lens group whose number of construction lenselements is the largest among the construction lens groups. Inparticular, the positive lens G24 positioned on the most reductionconjugate side is a lens which was produced through molding of a plasticmaterial and whose both surfaces each have an aspherical surface shape.With this construction, it becomes possible to mainly compensate fordistortion, curvature of field, and the like with efficiency.

In this embodiment, temperature drift characteristics at the time ofenvironmental variations are mutually canceled between the positive lensG24 made of a plastic material and the negative lens G11 made of aplastic material in the first lens group L1.

The second lens group L2 is set so that the magnification •_(VW) in thetotal magnification changing region exists in the vicinity ofapproximately equal-magnification.

The third lens group L3 includes one positive lens G31. The positivelens G31 is used mainly for a refractive action where off-axis mainlight beam angle is set closer to parallel to the optical axis and forreduction in synthesized refractive power of the first lens group L1 andthe second lens group L2. As a material of the positive lens G31,s-lal14 (manufactured by Kabushiki Kaisha Ohara) is used. Preferably,when refractive indexes with respect to d-, g-, F-, and c-lines arerespectively referred to as “nd”, “ng”, “n_(F)”, and “nc” and thefollowing equations are satisfied, glass satisfying the followingexpression is selected.υ_(d)=(n _(d)−1)/(n _(F) −n _(C))•_(gF)=(n _(g) −n _(F))/(n _(F) −n _(C))•_(gF)=(0.6438−0.001682υ_(d))>0In this case, correction of chromatic aberration of magnification in awide band becomes easy.

According to this embodiment, it becomes possible to realize a zoom lensthat has a large aperture (F-value=1.7) and high luminance but has lessfocus movements at the time of magnification changing.

Also, numerical values of Conditional Expressions (1) to (5) in thisembodiment are given below.

Conditional Expressionsβ_(VW)=−0.94  (1)L/f _(r)=0.87  (2)2F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(r) ² /√{square rootover (Z)}=0.080  (3)2·F _(T) ·ε−|BF _(W) −BF _(T)|=0.055  (4)F_(m)=1.84  (5)

SECOND EMBODIMENT

The second embodiment in FIG. 3 has a three-group construction includingthe lens group L1 of negative refractive power, the lens group L2 ofpositive refractive power, and the lens group L3 of positive refractivepower in this order from the enlargement conjugate side to the reductionconjugate side. When an object-image distance is changed, focusadjustment is performed by moving the first lens group L1.

At the time of magnification changing from the wide angle end to thetele photo end, the first lens group L1 and the third lens group L3 arefixed and have constant total lengths and only the second lens group L2is independently moved toward the enlargement conjugate side. Also, inorder to ensure illuminance on the screen S, a multilayer coat isapplied to each lens surface.

The first lens group L1 has a three-lens construction including anegative lens G11, a negative lens G12, and a positive lens G13 from theenlargement conjugate side. By designing the first lens G11 disposed onthe most enlargement conjugate side using a negative lens, an apparentpupil position is set on the enlargement conjugate side and the diameterof a front lens is reduced. Also, the first lens G11 is a lens which wasproduced using special low dispersion glass “s-FPL51” (manufactured byKabushiki Kaisha Ohara) with an Abbe number of 80 or more and whose bothsurfaces each have an aspherical surface shape. With this construction,it becomes possible to compensate for distortion and the like withefficiency. In particular, the aspherical surface shape is provided fora concave surface on the reduction conjugate side and has a shape wherethe concave degree is reduced in a direction from the lens center to thelens peripheral portion. With this construction, it becomes possible tocompensate for distortion, introversive coma aberration, and the likewith efficiency.

The second lens group L2 functions as a magnification changing lensgroup. In this embodiment, the second lens group L2 includes a positivelens G21, a diaphragm SP, a negative lens G22, and a positive lens G23in this order from the enlargement conjugate side, and is a lens groupwhose number of construction lens elements is the largest among theconstruction lens groups. In particular, the positive lens G23positioned on the most reduction conjugate side is a lens which wasproduced through molding of a glass material and whose both surfaceseach have an aspherical surface shape. With this construction, itbecomes possible to mainly compensate for distortion, curvature offield, and the like with efficiency. In particular, the asphericalsurface shape is provided for a convex surface on the reductionconjugate side and has a shape where the convex degree is reduced in adirection from the lens center to the lens peripheral portion. With thisconstruction, it becomes possible to favorably compensate fordistortion, curvature of field, and the like.

The second lens group L2 is set so that the magnification •_(VW) in thetotal magnification changing region exists in the vicinity ofapproximately equal-magnification.

Other points are the same as those in the first embodiment, so thedetailed description thereof will be omitted.

According to this embodiment, it becomes possible to realize a zoom lensthat has a large aperture (F-value=1.7) and high luminance but has lessfocus movements at the time of magnification changing.

Also, numerical values of Conditional Expressions (1) to (5) in thisembodiment are given below.

Conditional Expressionsβ_(VW)=−0.94  (1)L/f _(r)=0.54  (2)2F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(r) ² /√{square rootover (Z)}=0.076  (3)2·F _(T) ·ε−|BF _(W) −BF _(T)|=0.052  (4)F_(m)=1.80  (5)

THIRD EMBODIMENT

The third embodiment shown in FIG. 5 has a four-group constructionincluding the lens group L1 of negative refractive power, the lens groupL2 of positive refractive power, the lens group L3 of positiverefractive power, and the lens group L4 of positive refractive power inthis order from the enlargement conjugate side to the reductionconjugate side. When an object-image distance is changed, focusadjustment is performed by moving the second lens group L2.

At the time of magnification changing from the wide angle end to thetele photo end, the first lens group L1, the second lens group L2, andthe fourth lens group L4 are fixed and have constant total lengths andonly the third lens group L3 is independently moved toward theenlargement conjugate side. Also, in order to ensure illuminance on thescreen S, a multilayer coat is applied to each lens surface.

The first lens group L1 has a two-lens construction including a negativelens G11 and a negative lens G12 from the enlargement conjugate side. Bydesigning the first lens G11 disposed on the most enlargement conjugateside using a negative lens, an apparent pupil position is set on theenlargement conjugate side and the diameter of a front lens is reduced.

Also, like in the second embodiment, the first lens G11 is a lens whichis made of special low dispersion glass “s-FPL51” (manufactured byKabushiki Kaisha Ohara) with an Abbe number of 80 or more and whose bothsurfaces each have an aspherical surface shape. With this construction,it becomes possible to compensate for distortion and the like withefficiency.

The second lens group L2 performs focusing when an object-image distanceis changed. With this construction, it becomes possible to realize azoom lens where even when a throwing distance is changed, lessaberration variations occur.

The third lens group L3 and the fourth lens group L4 are the same as thesecond lens group L2 and the third lens group L3 in the secondembodiment.

According to this embodiment, it becomes possible to realize a zoom lensthat has a large aperture (F-value=2.0) and high luminance but has lessfocus movements at the time of magnification changing.

Also, numerical values of Conditional Expressions (1) to (5) in thisembodiment are given below.

Conditional Expressionsβ_(VW)=−0.91  (1)L/f _(r)=0.51  (2)2F _(m) ·ε−|−f _(V)·(1−√{square root over (Z)})²|·β_(r) ² /√{square rootover (Z)}=0.061  (3)2·F _(T) ·ε−|BF _(W) −BF _(T)|=0.060  (4)F_(m)=2.08  (5)

Here, specific values of the variables described above in numericalembodiments are given below.

TABLE 1 FIRST SECOND THIRD NUMERICAL NUMERICAL NUMERICAL EMBODIMENTEMBODIMENT EMBODIMENT Fm 1.84 1.80 2.08 FT 1.98 1.87 2.16 • 0.028 0.0280.028 fv 47.17 44.96 43.73 Z 1.10 1.10 1.15 •r 0.459 0.493 0.511 BFw6.604 6.591 6.601 B = T 6.604 6.591 6.601

As described above, according to each embodiment, it becomes possible toobtain a zoom lens that favorably corrects various aberrations in anentire magnification changing region, achieves favorable opticalperformance across the whole of a screen, and is suitable for a liquidcrystal projector.

Hereinafter, first to third numerical embodiments corresponding to thefirst to third embodiments of the present invention will be described.In the numerical embodiments, “i” represents the order of acorresponding optical surface from the enlargement conjugate side.Therefore, “Ri” denotes the radius of curvature of the ith opticalsurface, “di” denotes the interval between the ith surface and the i+1thsurface, and “ni” and “vi” respectively denote the refractive index andthe Abbe number of the material of the ith optical member with respectto the d-line.

Also, “f” is a focal distance, “FNO” is an F-number, and “•” is a halfangle of view. The last five optical surfaces are surfaces constitutingthe glass block.

In the aberration diagrams, “B”, “G”, and “R” respectively denote 470nm, 550 nm, and 650 nm and “•M” and “•S” respectively represent ameridional image surface and a sagittal image surface.

When a displacement in the optical axis direction at a position with aheight h from the optical axis is referred to as “X” by setting asurface vertex as a reference, the aspherical surface shape is expressedas follows.

$x = {\frac{\left( {1/R} \right)h^{2}}{1 + \sqrt{\left\{ {1 - {\left( {1 + k} \right)\left( {h/R} \right)^{2}}} \right\}}} + {A\; h^{2}} + {Bh}^{4} + {Ch}^{6} + {Dh}^{8}}$Here, “R” is a paraxial radius of curvature, “k” is a conical constant,and “A”, “B”, “C”, “D”, and “E” are each an aspherical surfacecoefficient.

Also, “e-X” means “×10^(−X)”.

TABLE 2 First Numerical Embodiment f: 18.8 mm to 20.7 mm FNO: 1.74 to1.98 ω: 26.82° to 24.67° R d n ν 1 ( ) 1.50 1.532 55.8 2 ( ) 7.61 3−25.913 1.50 1.705 41.2 4 27.000 2.35 5 37.017 3.89 1.839 37.2 6 −46.758( ) 7 29.129 3.13 1.792 47.4 8 158.399 18.90 9 −18.448 1.10 1.812 25.410 181.242 0.20 11 49.113 6.93 1.605 60.6 12 −20.862 0.75 13 ( ) 2.881.532 55.8 14 ( ) ( ) 15 38.093 4.01 1.699 55.5 16 1621.356 4.05 17 ∞23.00 1.518 64.1 18 ∞ 0.44 1.502 65.0 19 ∞ 0.50 1.767 65.0 20 ∞ 2.201.462 65.0 21 ∞ Inter-Group Data W T d6 18.22 13.71 d14 0.74 5.26Aspherical Surface Data c(1/r) k A B C D 1 4.780e−002 7.993e−001−2.120e−005 6.817e−008 1.227e−010 4.360e−013 2 8.568e−002 −2.824e−001−2.944e−005 8.476e−008 −5.653e−010 1.301e−011 13 −2.814e−003 5.630e+0021.257e−005 −7.146e−008 −1.637e−010 −2.250e−013 14 −1.477e−002 1.728e+0006.616e−006 −2.690e−008 −2.181e−010 5.907e−013

TABLE 3 Second Numerical Embodiment f: 18.8 mm to 20.7 mm FNO: 1.74 to1.87 ω: 26.81° to 24.67° R d n ν 1 ( ) 1.50 1.498 81.5 2 ( ) 12.94 3−17.752 2.00 1.699 55.5 4 41.406 1.21 5 44.846 4.22 1.703 48.1 6 −27.447( ) 7 26.107 2.67 1.776 49.6 8 99.214 19.72 9 −15.932 1.10 1.791 25.7 10124.414 0.20 11 ( ) 8.29 1.696 53.2 12 ( ) ( ) 13 40.202 3.46 1.699 55.514 1707.641 4.05 15 ∞ 23.00 1.518 64.1 16 ∞ 0.44 1.502 65.0 17 ∞ 0.501.767 65.0 18 ∞ 2.20 1.462 65.0 19 ∞ Inter-Group Data W T d6 10.70 6.42d12 4.77 9.05 Aspherical Surface Data c(1/r) k A B C D 1 4.911e−0027.993e−001 −2.120e−005 6.817e−008 1.227e−010 4.360e−013 2 8.558e−002−2.824e−001 −2.944e−005 8.476e−008 −5.653e−010 1.301e−011 11 2.459e−002−1.242e+001 9.684e−006 8.015e−009 −8.876e−011 5.887e−013 12 −5.652e−002−3.838e−002 8.653e−006 4.445e−008 −3.024e−010 1.247e−012

TABLE 4 Third Numerical Embodiment f: 18.6 mm to 21.5 mm FNO: 2.00 to2.16 ω: 27.01° to 23.86° r d nd νd 1 ( ) 1.50 1.498 81.5 2 ( ) 12.85 3−17.760 2.00 1.699 55.5 4 44.220 ( ) 5 46.962 4.15 1.703 48.1 6 −27.800( ) 7 25.777 2.70 1.776 49.6 8 102.837 19.33 9 −15.864 1.10 1.791 25.710 116.904 0.20 11 ( ) 8.30 1.696 53.2 12 ( ) ( ) 13 41.476 3.31 1.69955.5 14 1649.328 4.05 15 ∞ 23.00 1.518 64.1 16 ∞ 0.44 1.502 65.0 17 ∞0.50 1.767 65.0 18 ∞ 2.20 1.462 65.0 19 ∞ Inter-Group Data W T d4 1.151.15 d6 11.20 4.99 d12 3.84 10.04 Aspherical Surface Data c(1/r) k A B CD 1 4.911e−002 7.993e−001 −2.120e−005 6.817e−008 1.227e−010 4.360e−013 28.577e−002 −2.824e−001 −2.944e−005 8.476e−008 −5.653e−010 1.301e−011 112.574e−002 −1.200e+001 9.540e−006 7.236e−009 −9.283e−011 5.912e−013 12−5.679e−002 −4.315e−002 8.900e−006 4.606e−008 −3.042e−010 1.231e−012

FIG. 7 is a schematic view showing a main portion of an embodiment wherethe zoom lens according to the present invention is used.

In the drawing, an image projection apparatus is shown in which the zoomlens described above is applied to a three-panel color liquid crystalprojector, image information of light in multiple colors based onmultiple liquid crystal display elements is synthesized through a colorsynthesis means, and magnification and projection onto the screensurface is performed using the zoom lens. In FIG. 7, a color liquidcrystal projector 1 synthesizes light in respective colors of RGB fromthree liquid crystal panels 5B, 5G, and 5R for RGB into one optical pathusing a prism 2 serving as the color synthesis means (the presentinvention is not limited to the synthesis using one prism and the colorsynthesis may be performed using multiple dichroic mirrors, dichroicprisms, or multiple polarizing beam splitters) and throws thesynthesized light onto a screen 4 using a throwing lens 3 composed ofthe zoom lens described above. Here, it does not matter whether theliquid crystal display elements are transmission-type liquid crystaldisplay elements or reflection-type liquid crystal display elements.Also, there occurs no problem even when only one liquid crystal displayelement is used. Further, the liquid crystal display elements may bechanged into mirror devices such as DMDs.

Also, in this embodiment, a construction has been described in whichonly one lens group is moved at the time of magnification changing,however as a matter of course, two lens groups adjacent to each other ortwo lens groups separated from each other may be moved integrally.

According to the embodiment described above, it becomes possible toobtain a zoom lens that favorably corrects various aberrations in anentire magnification changing region using a simple lens construction,achieves favorable optical performance across the whole of a screen, andis suitable for a projection optical system of a liquid crystalprojector or the like.

1. A zoom lens comprising: three lens units including a first lens unithaving negative refractive power, a second lens unit having positiverefractive power, and a third lens unit having positive refractive powerin this order from an enlargement conjugate side, wherein amagnification change is performed by moving only the second lens unit inan optical axis direction of the zoom lens, focus adjustments of thezoom lens is performed by moving the first lens unit on the opticalaxis, and the zoom lens is approximately telecentric on reductionconjugate side of the zoom lens, wherein when a focal distance of thesecond lens unit is referred to as “f_(v)” and a lateral magnification,a magnification changing ratio, an F-value in an intermediate region ata time of the magnification changing, and a minimum diameter of a circleof confusion of the third lens unit are respectively referred to as“β_(r)”,“Z”, “F_(m)”, and “ε”, a following expression is satisfied:2F _(m) ·ε−|−f _(v)·(1−√Z)²|·β_(r) ² /√Z>0.
 2. A zoom lens according toclaim 1, wherein a position of the second lens unit at a tele photo endis on the enlargement conjugate side with respect to a position of thesecond lens unit at a wide angle end.
 3. A zoom lens according to claim1, wherein when an imaging magnification at a magnification changingposition at a wide angle end of the second lens unit is referred to as“β_(vw)”, a following expression is satisfied:−1.2<β_(vw)<−0.8.
 4. A zoom lens according to claim 1, furthercomprising an aperture diaphragm, wherein when a synthesized focaldistance from the aperture diaphragm to a lens system on the reductionconjugate side is referred to as “f_(r)” and a distance from theaperture diaphragm to a principal point position on an enlargementconjugate side of the lens system on the reduction conjugate side isreferred to as “L”, a following expression is satisfied:0.45<L/f _(r)<1.1.
 5. A zoom lens according to claim 1, wherein whenparaxial back focuses at magnification changing positions at a wideangle end and a tele photo end are respectively referred to as “BF_(w)”and “BF_(T)”, an F-value at the tele photo end is referred to as“F_(T)”, and a minimum diameter of a circle of confusion is referred toas “ε”, a following expression is satisfied:2·F _(T) ·ε−|BF _(W) −BF _(T)|>0.
 6. A zoom lens according to claim 1,further comprising an aperture diaphragm, wherein one or more surfaceseach in an aspherical surface shape exist on an enlargement conjugateside and the reduction conjugate side with the aperture diaphragmin-between.
 7. A zoom lens system according to claim 1, furthercomprising an aperture diaphragm, wherein a negative lens including asurface in an aspherical surface shape and a positive lens including asurface in an aspherical surface shape exist respectively on anenlargement conjugate side and the reduction conjugate side with theaperture diaphragm in-between.
 8. A zoom lens system according to claim7, wherein the surface in the aspherical surface shape of the negativelens has a concave shape whose concave degree is reduced in a directionfrom a lens center to a lens peripheral portion, and the surface in theaspherical surface shape of the positive lens has a convex shape whoseconvex degree is reduced in a direction from a lens center to a lensperipheral portion.
 9. A zoom lens according to claim 7, wherein thenegative lens and the positive lens are each a lens made of plastic. 10.An image projection apparatus comprising: an image display element; anda zoom lens according to claim 1 for projecting light from the imagedisplay element.
 11. A zoom lens comprising: a first lens unit havingnegative refractive power; a second lens unit having positive refractivepower; and a third lens unit having positive refractive power, whereinthe first lens unit, the second lens unit, and the third lens unit arelocated in this order from an enlargement conjugate side, whereinmagnification changing is performed by moving only the second lens unitand focus adjustment is performed by moving only the first lens unit,and wherein when a focal distance of the second lens unit is referred toas “f_(v)” and a lateral magnification, a magnification changing ratio,an F-value in an intermediate region at a time of the magnificationchanging, and a minimum diameter of a circle of confusion of the thirdlens unit are respectively referred to as “β_(r)”, “Z”, “F_(m)”, and“ε”, a following expression is satisfied:2F _(m) ·ε−|−f _(v)·(1−√Z)²|·β_(r) ² /√Z>0.
 12. An image projectionapparatus comprising: an image display element; and a zoom lens forprojecting light from the image display element, wherein zoom lenscomprises a first lens unit having negative refractive power, a secondlens unit having positive refractive power, and a third lens unit havingpositive refractive power, which are located in this order from anenlargement conjugate side, wherein magnification changing is performedby moving only the second lens unit and focus adjustment is performed bymoving only the first lens unit, and wherein when a focal distance ofthe second lens unit is referred to as “fv” and a lateral magnification,a magnification changing ratio, an F-value in an intermediate region ata time of the magnification changing, and a minimum diameter of a circleof confusion of the third lens unit are respectively referred to as“β_(r)”, “Z”, “F_(m)”, and “ε”, a following expression is satisfied:2F _(m) ·ε−|−f _(v)·(1−√Z)²|·β_(r) ² /√Z>0.